Method for manufacturing photovoltaic panels by the use of a polymeric tri-layer comprising a composite getter system

ABSTRACT

The present invention relates to a method for manufacturing thin-film photovoltaic panels by the use of a sealing means composed by a polymeric tri-layer comprising a composite getter system composed of a polymer with a low H 2 O transmission, having dispersed in its inside a H 2 O sorption material, and two outer polymeric layers with the composite getter system therebetween, as well as to a polymeric tri-layer for the manufacturing of photovoltaic panels.

The present invention refers to a method for manufacturing photovoltaicpanels by means of a polymeric tri-layer comprising a composite gettersystem for sorbing H₂O.

The terms photovoltaic and solar could be used to indicate the same orequivalent type of devices (panel, cells) and are to be consideredtechnically equivalent, even though in the following it will bepreferably employed the term photovoltaic panel.

A photovoltaic panel is composed of one or more photovoltaic elements(so-called cells) for the conversion of solar energy into electricenergy. By the term photovoltaic cell reference is made to a singleactive element, that is the element that converts the light radiationinto electric energy, while by photovoltaic panel reference is made tothe final product, that is to the set of suitably interconnectedphotovoltaic cells, provided with electrical connections and finallyencapsulated. A photovoltaic panel, sometimes also referred to in thefield by the term photovoltaic module, may contain more than onephotovoltaic cell (the number of photovoltaic cells can vary from 50 to150). Typically in a thin-film panel the number of cells is about 80.

There are various types of photovoltaic cells. Among those recentlydeveloped, the thin-film photovoltaic cells are of particular interest,due to their conversion efficiency and industrial feasibility. In thesecells the active element is deposited in form of a film on a (vitreous,metallic or plastic) substrate and is not present in the form ofexpensive stripes or slices as in the case of mono- or polycrystallinesilicon photovoltaic cells. In these cells are also presentmetallizations placed in contact with the active element, having thefunction of interconnecting the cells as such and transporting thecurrent generated by them.

Among the most interesting typologies of photovoltaic cells there arethe cells based on cadmium-tellurium, amorphous silicon,copper-indium-selenium having gallium and sulphur added and the cellsbased on gallium arsenide. More information on the different typologiesof photovoltaic cells and their functioning can be found in the article“Prowess in Amorphous Silicon Based Solar Cell Technology”, by Wronskiet al., presented at “World Climate & Energy Event” in 2002, in thearticle “Status of Amorphous and Crystalline Thin-Film Silicon SolarCell Activities”, by Bolk von Roedern, presented in 2003 at “NCPV andSolar Review Meeting” and in the article “Development of Thin-FilmCu(In,Ga)Se₂ and CdTe Solar Cells” by Romeo et at., published in“Progress in Photovoltaics: Research and Application” in 2004, volume12, pp 93-111.

The final structure of the solar panel is rather standardized andindependent on the specific type of photovoltaic cell and foresees theuse of two, vitreous or plastic, supports to confine and enclose thephotovoltaic element. These supports, one of which necessarily must betransparent to the light radiation, ensure also the mechanical stabilityand the protection from the atmospheric agents.

These supports are typically joined together by arranging anencapsulating polymer having good adhesive properties in the spacebetween the two supports; in some cases there can also be spacingelements that fix the distance between the supports, while in othercases it is the thickness of the polymer to determine the distancebetween the supports. The method of the invention is applied to thelatter type of structure.

In the following the supports are identified and distinguished betweeneach other by the use of the terms “upper support” that refers to thesupport through which the radiation reaches the cell, and “lowersupport” that identifies the support on the back of the cell.

The photovoltaic element can be directly in contact with one of theinner surfaces of the supports of the photovoltaic module or can becompletely encapsulated by a transparent, polymeric material having alow transmission for H₂O, that is a transmission lower than 10 g m-2 d-1mm (grains of H₂O per square meter per day for each millimetre of widthof the material) at 25° C. and 60% of relative humidity (RH). In thetechnical field the transmission of water by the polymer can also becharacterized by means of the MVTR, standing for Moisture VapourTransmission Rate; the two are strictly correlated, the permeabilitybeing the MVTR multiplied by the thickness of the polymeric, materialand divided by the pressure.

The polymeric material used for encapsulating the photovoltaic elementis typically composed of EthylVinylAcetate (EVA); often used are alsothermoplastic polyurethanes (TPU) and PolyVinylButyral (PVB). Thispolymeric material has essentially the aim to fill the inner volume ofthe photovoltaic panel giving to it at the same time also mechanicalstability. By inner volume of the photovoltaic panel is intended thevolume that is defined by the two supports and by the frame of the panel(typically formed of a polymer having good adhesive properties) that isnot occupied by the photovoltaic element or by the other constructiveelements of the photovoltaic panel (for example the electric contacts).If the photovoltaic element is in contact with one of the two supports(typically the lower one), it finds itself encapsulated on three sidesby the encapsulating polymeric material.

The manufacturing process of a photovoltaic panel foresees also aprocess of thermosealing. In this regard two main types of processes areused, one foresees the use of a vacuum laminator, while the otherforesees the use of an autoclave, In both cases the thermosealing isusually carried out between 100 and 170° C. This process results in themelting of the encapsulating polymer.

Independently from the specific type of photovoltaic cell, the presenceof H₂O inside the photovoltaic panel deteriorates the characteristicsthereof; the deterioration mechanisms triggered by the presence of H₂Oact both at cell level, and at the solar module level. As far as thecell is concerned, the deterioration is due to the oxidation and thecorrosion of the thin films that form the cell, while as far as themodule is concerned, there is the corrosion of the metallizations usedfor the electric connections. Further information in this respect can befound in the Ph.D. thesis “Stability Diagnostics for Thin-FilmPhotovoltaic Modules” by T. Carlsson of Helsinki University, publishedin 2006, and in the Ph.D. thesis “Design and Stability ofCu(Tn,Ga)Se₂-Based Solar Cell Modules” by J. Wannerberg of UppsalaUniversity, published in 2006.

The peripheral adhesion region, between the upper support and the lowersupport, which can also be seen as the frame of the panel, represents apreferential region for the ingress of water inside the device since thetwo supports, upper and lower one, are to be considered, in this case,impermeable for H₂O.

The problem of the presence of H₂O inside the photovoltaic panel hasbeen confronted according to three main approaches: introduction of aH₂O sorbing material inside the panel; use of a barrier having low H₂Otransmission; use of a barrier having low H₂O transmission andcontaining also a H₂O sorbing element.

As far as the first type of solution is concerned, that is the inclusionof a sorbing material inside the photovoltaic cell or panel, this isdescribed in the Japanese patent JP 2800528 B2, in which the use ofvarious possible water sorbers is described, that are positioned in thephotovoltaic cell in peripheral regions of the lower surface thereof Inthis document the getter element is totally independent from. theproblem of sealing the photovoltaic panel.

The use of barriers having low H₂O transmission, comprising a H₂O gettermaterial, is described in the patent application EP 1617494 A2, wheresuch a barrier replaces the upper support of the photovoltaic panel.

The use of adhesives arranged in peripheral regions of the panel to makethe upper and lower support stick to each other is described in thepatent application WO 2004/019421, that specifies special types ofadhesives having low H₂O transmission, while the document WO 03/050891describes sealing materials containing a. limited weight percentage,between 0.1% and 10%, of a H₂O sorber.

The main problem associated to the use of barriers or adhesives withoutgettering capacity is that, even if slowly, the ingress of H₂O in thephotovoltaic panel occurs in a continuous and progressive manner, thuscausing a gradual deterioration of the characteristics of the panel.

This problem tends to show up after a relatively short time intervalalso in the case in which adhesives or sealing materials are used thatcomprise a limited quantity of getter material.

The use of a sealing element arranged along the edges of the panelhaving both the function of adhesive between the supports and of barrieragainst the entry of H₂O and H₂O sorber, entails notable restrictions onthe final characteristics of this element and results in non-optimalcompromise choices, for example limiting the quantity of sorbingmaterial present in the composite getter system in order not to affectthe adhesion of the composite getter system to the two supports.

Object of the present invention is to provide an improved method formanufacturing photovoltaic panels, capable of separating the problem ofadhesion from the problem of H₂O sorption and thus in a first aspectthereof it consists in a method for manufacturing a photovoltaic panelcomprising one or more photovoltaic elements confined by two supports,characterized in. that said supports, before being subjected to athermosealing process, are joined together in proximity of the edge bymeans of three superimposed polymeric layers, in Which the two outermostlayers are composed of a polymeric material essentially without gettermaterial, whereas the central layer is composed of a composite gettersystem for H₂O sorption.

The invention will be illustrated, in the following with reference tothe drawings, in which:

the FIGS. 1A and 1B represent a cross-sectional view of a photovoltaicpanel manufactured according to the present invention, respectivelybefore (1A) and after (1B) the thermosealing process;

FIG. 2 shows an enlarged detail of a polymeric tri-layer according tothe present invention after being subjected to the thermosealingprocess; and

the FIGS. 3A-3C represent EDX spectra that show the intensity of thecalcium. peak in various points of the tri-layer after the thermosealingprocess.

In the drawings the dimensions and the dimensional relations between.the various elements, particularly with reference to their thicknesses,are not correct, but have been distorted for the sake of bettercomprehensibility of the figures. Further some constitutive elements ofa photovoltaic panel, as for example electrical feedthroughs andconnections, have not been shown because they are not relevant for thedescription of the present invention.

In the realization of the polymeric tri-layer object of the presentinvention is important that the two outermost polymeric layers areessentially free of H₂O sorbing material, meaning that the amount ofsorbing materials or moisture getters in such layers is not higher than1 wt %. This means that the presence of small amounts of sorbingmaterial may be tolerated since not impairing the adhesive properties ofsaid outermost polymeric layers.

FIG. 1A shows in cross section a photovoltaic panel 10, to which themethod of the invention is applied, before the operation ofthermosealing of the supports. In this drawing the photovoltaic element11 has been shown in contact with the lower support 12. This element isencapsulated by a polymeric material 14 that fills the inner volume ofthe photovoltaic panel delimited by the lower support 12, by the uppersupport 13 and by the tri-layer formed of the composite getter system 15and of the outer polymeric layers without getter material 16, 16′; thistri-layer is arranged along the edge of the panel.

The polymer used for the outer layers 16, 16′ of the tri-layer must havegood adhesive properties towards the materials that form the upper andlower support of the photovoltaic panel, i.e. have an adhesion to thesupports of the panel quantifiable by a shear force resistance of atleast 100 KPa as measured according to the ASTM C961-06 procedure.

The information on the lap shear force (often referred also as lap shearstrength) of different materials is readily available to a personskilled in the art and could be easily retrieved by a plurality ofsources, see for example the publication “Moisture transport, adhesionand corrosion protection of PV module packaging materials” by Jorgensenet al, published in 2006 in the Solar energy Materials & solar Cells,pages 2739-2775, or the 2007 edition of the “Physical Properties ofPolymers Handbook” edited by Springer New York; moreover the informationon the lap shear force of a material to be employed in a productionprocess of photovoltaic elements is often specified by the materialsupplier.

Typical materials for the manufacturing of the transparent support arevitreous materials, whereas the other support of the panel can be madeof various materials, among the most common ones of which there arevitreous materials; if this support should also have the characteristicof flexibility, an aluminized polymeric foil could be used. For example,for the polymeric foil, polyvinyl fluorides can be used, among which theuse of Tedlar® is preferred, both used alone or in combination withother elements as polymeric foils, or aluminized polyethyleneterephthalate (PET).

FIG. 1B shows the photovoltaic panel 10 after the process ofthermosealing. In this case the constituent elements of the tri-layer,i.e. the outer polymeric layers 16, 16′ and the composite getter system15 arranged in the centre of the tri-layer, soften and melt into eachother and give rise to a composite 17 inside of which the H₂O sorber isdistributed.

This new composite getter system 17 that is formed following thesoftening and melting process shows a good adhesion to the supports ofthe photovoltaic panel in so far as the adhesive characteristics aredetermined essentially by those of the outer layers of the tri-layer,whereas the diffusion of the sorbing material ensures the formation of abarrier active against the ingress of H₂O inside the panel.

The thermosealing of the panel occurs by heating at a temperaturebetween 100° C. and 170° C. The manufacturing process of a photovoltaicpanel foresees also a process of thermosealing, in this regards two maintypes of processes are used, one foresees the use of a vacuum laminator,while the other foresee the use of an autoclave. In both cases thethermosealing is usually carried out between 100 and 170° C. Thisprocess results in the softening or melting of the encapsulatingpolymer.

As shown in FIG. 1B, as a result of this operation it is possible thatthe thickness of the composite getter 17 obtained by the softening,melting and the reciprocal interdiffusion of the layers of the tri-layerresults smaller than the starting thickness of the polymeric tri-layeras a consequence of the combined action of the weight of the uppersupport and the heating. Further it is possible that the compositegetter 17 slightly stands out, both towards the inside and towards theoutside of the panel 10, in which case the quantity of material thatoutflows from the panel is removed. This effect may be present with boththe two different types of thermosealing processes.

With regard to the properties required for the used polymers, these musthave a softening or melting temperature lower than 170° C.; preferablythis temperature is lower than or equal to 150° C.; they must furtherhave a low MVTR for H₂O, preferably lower than 10 g m⁻² d⁻¹ mm at 25° C.and 100% relative humidity. Polymers useful for carrying out theinvention are for example EthylVinylAcetate (EVA), polyethylene of low(LDPE), medium (MDPE) and high (HDPE) density, polyether block amides(PEBA), ionomeric resins such as Surlyn™ commercialized, by DuPont,ethylene-acrylic, acid copolymers as for example Lucalen® commercializedby Basell, polyvinylidene fluoride (PVDF), PolyVinylButyral (PVB) andpolyvinylidene chloride (PVDC) such as Saran™ commercialized by DOWChemicals, Ethylene-Propylene Rubbers (EPR), Ethylene Propylene DieneMonomer Rubbers (EPDM) and butyl rubbers.

It is pointed out that the H₂O transmission, like also the softening ormelting temperature, are not linked univocally to the type of polymerbut are functions of various parameters, among which the composition isone of the most relevant ones; thus it is important not to choose thepolymer on the basis of the class it belongs to but on the basis of itsH₂O transmission and the softening or melting temperature, thisinformation being easily available to an expert in the field. Forexample, as far as the transmission characteristics of EVA areconcerned, this information is available in the article by Marais et al.“Permeation and Sorption of Water and Gases through EVA CopolymersFilms” published in 2002 in Mat. Res. Innovation, Vol. 6, pp. 79-88.

With regard to the characteristics of the polymers used for the outerlayers of the tri-layer, these must further have good adhesivecharacteristics to the supports of the photovoltaic panel, quantifiableby a shear force resistance measured according to the ASTM C961-06procedure of at least 100 KPa. In any case, independently by thespecific characterizing method adopted for assessing the lap shearresistance, it has to be intended that the good adhesion properties ofthe outer polymeric layers are triggered or enhanced by thermalprocesses, such as the one specified in ASTM C961-06 or as the onesderiving from the thermosealing process of the photovoltaic module.

The adhesive properties of these polymers can be improved by addingsuitable additives such as for example silanes or polyolefinesfunctionalized by maleic anhydride wafting.

In a preferred embodiment the tri-layer is composed of the same type ofpolymer, with the only difference that the outer polymeric layers of thetri-layer have added silanes to improve the adhesive characteristicsthereof in order to reach the lap shear resistance of at least 100 KPa.In an even more preferred embodiment this common polymeric material isthe same as the polymeric material used for encapsulating thephotovoltaic element; in this latter case the use of EVA result to bevery advantageous.

The above-mentioned supports of the photovoltaic panel can be made fromglass or, in the case it is desired to obtain flexible panels, can bemade from plastic materials such as ethylene tetrafluoroethylene (ETFE).

In the case in which the polymeric materials that form the initialtri-layer are different from each other, it is important that thepolymeric material of the composite getter system is, at its softeningor melting temperature, miscible with the polymeric material forming theouter layers of the tri-layer.

With regard to the materials for H₂O sorption are concerned, zeolites,silica gels, alumina and alkaline earth metal oxides can he used.

The composite getter system for the H₂O sorption that forms the centralpart of the tri-layer has a weight percentage of the H₂O sorbingmaterial comprised between 10 and 60 wt %, preferably comprised between30 and 45 wt %. It is important that the quantity of H₂O sorbingmaterial must be not less than 10% by weight, in order to have anefficient barrier. It is important to remember that this quantity bydiffusing also in the outer polymeric layers of the tri-layer during thethemosealing process creates the barrier active against the entry of H₂Ointo the photovoltaic cell, whereby the medium concentration of thesorbing material of the composite getter system 17 obtained after thesoftening and melting of the starting tri-layer is necessarily lowerwith respect to the starting composite getter system one, resulting inthe lower limit given above.

The inventors have determined that when operating under the conditionsof the invention the sorbing material is diffusing also in proximity ofthe supports of the photovoltaic panel, thus creating a barrier activeagainst the ingress of H₂O inside the device, without howevercompromising the adhesive characteristics that the interface between thetwo supports must have.

In a second aspect thereof the invention consists in a polymerictri-layer for the manufacturing of photovoltaic panels, characterized inthat the outermost are essentially without getter material, whereas theinnermost layer is a composite getter system composed of a polymer withinside a dispersion of a moisture getter material.

Moreover, the outermost layers have good adhesive characteristics,quantifiable by a shear force resistance of at least 100 kPa.

Example 1

A tri-layer is manufactured using two outer layers with a thickness of0.6 mm of EVA comprising an additive belonging to the family of silanesthat acts as adhesion promoter. The inner layer with a thickness of 1.4mm is composed of EVA loaded by 40% in weight with calcium oxide powder.The previous tri-layer is used as interface between two glass supportssimulating thus the structure of a photovoltaic panel. The sampleobtained in this way is subsequently subjected to a light compressionand a thermal process at 150° C. for 30 minutes, similar to that ofthermosealing in a photovoltaic panel. The shear force resistance forthis sample after a termosealing performed as described above is 5200KPa.

FIG. 2 shows the representation of a photograph of the contact regionbetween the lower support 12 and the tri-layer after that this has beensubjected to the thermal treatment of thermosealing that has caused thesoftening and melting of the layers, showing the parts without sorbingmaterial 14, and the parts 17 that contain said sorbing material.

It has been chosen to show the photographic representation and not toinclude the original photograph due to legibility problems connected tothe printing quality of the latter one.

Example 2

A sample prepared in a similar way to what described in example 1 isanalysed by means of an energy dispersive X-ray micro-analyser (OxfordEnergy Dispersive X-Ray Spectroscopy) connected to a scanning electronmicroscope (SEM Leica Stereoscan 420) in order to obtain a qualitativeand quantitative evaluation of the chemical composition of the samplesubjected to analysis. The spectra obtained in correspondence of thecentral section, of the upper support and of the lower support are shownrespectively in the FIGS. 3A, 3B, 3C.

Example 3

A tri-layer is manufactured in a manner similar to sample 1 but using ascentral layer MDPE loaded with 35% in weight with calcium oxide powder.The thickness of the central layer is 0.75 mm. Also this tri-layer isused as interface between two glasses supports simulating thus thestructure of a photovoltaic panel, The sample obtained in this way issubjected to a light compression and a thermal process at 150° C. for 30minutes, similar to that of thermosealing in a photovoltaic panel.

The shear force resistance for this sample after a thermosealingperformed as described above is 4300 KPa.

Example 4 (Comparative)

A 2 mm mono-layer is manufactured in a manner similar to the centrallayer of example 3. The higher thickness of this example enables to makea direct comparison with example 3 tri-layers. This monolayer is used asinterface between two glasses supports simulating thus the structure ofa photovoltaic panel. The sample obtained in this way is subjected to alight compression and a thermal process at 150° C. for 30 minutes,similar to that of thermosealing in a photovoltaic panel.

In this case the sample did now show any adhesive feature and the twoglasses were easily separated.

FIG. 2 shows that operating under the conditions of the invention thesorbing material diffuses also in correspondence of the vitreoussupport, i.e. it is possible, starting from an initial situation inwhich the concentration of the sorbing material is highly non-uniform(being present only in the central region of the tri-layer), to arriveat a situation where the sorbing material is diffused also incorrespondence of the vitreous surfaces, thereby eliminating thepreferential regions for the ingress of H₂O in the system. Theefficiency of this solution can actually be deduced from FIG. 2, whereit can be seen how the particles of the H₂O sorber (in the parts 17)arrive in contact with the vitreous support, thus eliminatingpreferential paths for the ingress of H₂O.

The FIGS. 3A, 3B, 3C show that there are significant quantities ofsorber, represented by the calcium peak, also in correspondence to thesupports, as confirmation of the fact that the method of the inventionallows to obtain a composite getter with good barrier properties alongthe total of its thickness from the softening and melting of a startingpolymeric tri-layer characterized by the presence of a composite getterlayer arranged in its central part.

1.-14. (canceled)
 15. A polymeric tri-layer for the manufacturing ofphotovoltaic panels, wherein the two outermost layers are composed of apolymeric material essentially without getter material, whereas thecentral layer is composed of a composite getter system for H₂O sorption.16. The polymeric tri-layer according to claim 15, in which said outerpolymeric layers have a shear force resistance of at least 100 KPa. 17.The polymeric tri-layer according to claim 15, in which said outerpolymeric layers are made from Butyl Rubber and said central layer ismade from EthylenePropylene Rubber.